Lanthanides in granulometric fractions of Mediterranean soils. Can they be used as fingerprints of provenance?

European Journal of Soil Science, March 2019, 70, 394–410 doi: 10.1111/ejss.12730 Lanthanides in granulometric fractions of Mediterranean soils. Can they be used as fingerprints of provenance? J. M. Martín-Garcíaa,A.Molinero-García a, J. Calerob, M. V. Fernández-Gonzáleza, J. Párragaa& R. Delgadoa aDepartamento de Edafología y Química Agrícola, Universidad de Granada, Campus Cartuja, 18071, Granada, Spain, and bDepartamento de Geología, Universidad de Jaén, Campus Las Lagunillas, 23071, Jaén, Spain Summary There is geochemical interest in the lanthanides because they behave like a group that is closely related to the parent materials during surface processes, although they also undergo fractionation as a result of supergene dynamics. We analysed lanthanide concentrations (ICPms) in the granulometric fractions fine sand, clay and free forms of clay (FFclay-CDB and FFclay-Ox: extracted with citrate-dithionite-sodium bicarbonate and with ammonium oxalate, respectively) from a soil chronosequence of Mediterranean soils. There was a relative enrichment of heavy rare earth elements (HREE) in the clay fraction and its free forms with respect to fine sand. The clay free forms behaved as scavengers of lanthanides, and oxidative scavenging of cerium (Ce) in FFclay-CDB was also detected. Lanthanide concentrations (lanthanum to gadolinium in fine sand; terbium to lutetium in clay) varied with soil age, and chronofunctions were established. There was a strong positive collinearity between most of the lanthanide concentrations. Furthermore, the value of the correlation index (Pearson’s r) of the concentrations between couples of lanthanides (rCLC) decreased significantly with increasing separation between the elements in the periodic table; this has never been described in soils. Several geochemical properties and indices in the fine sand and clay soil fractions and in the geological materials of the Guadalquivir catchment showed, on the one hand, a genetic relation between them all, enabling the lanthanides to be used as fingerprints of provenance; on the other hand, fractionation between fine sand and clay showed these are actively involved in soil lanthanide dynamics. Highlights •Are lanthanides from fine sand and clay genetically related to the geological materials? •Lanthanide concentrations of fine sand and clay fit chronofunctions •Pearson’s rof lanthanide couples decreases when separation increases in the periodic table •Free forms of clay are scavengers of lanthanides and concentrate HREE and cerium Introduction Quantities of rare earth elements (REE) (lanthanoids: 57-71Ln, and scandium 21Sc and yttrium 39Y) in the Earth’s crust are of the order mg kg−1, and show some characteristic periodic behaviour such as: (i) light REE (LREE), low atomic weight (Ar) (lanthanum La to samarium Sm), are more abundant than heavy REE (HREE) (holmium Ho to lutetium Lu) (British Geological Survey, 2011) (medium REE, MREE: samarium Sm to dysprosium Dy, have an intermediate Ar; Rollinson, 1993) and (ii) a strong linear correlation Correspondence: R. Delgado. E-mail: [email protected] Received 26 February 2018; revised version accepted 11 July 2018 between their concentrations has been reported; in European soils, r>0.7 (Salminen, 2005). The chemical signatures of the parent rock assemblages in a tectonic province can persist in the daughter sediments produced, and are thus preserved in the corresponding sedimentary deposits (Rollinson, 1993). It has been suggested (Blundy & Wood, 2003) that the trace elements (<0.1% by weight, as in the case of lanthanides) exhibit passive behaviour during supergene processes, resulting in their being excellent tracers of the source area of sediments and soil materials. Wang et al. (2017) described them as ideal tracers of origin in aeolian research. Consequently, the geochemical interest in the lanthanides is because of their close relation 394 © 2018 The Authors. European Journal of Soil Science published by John Wiley & Sons Ltd on behalf of British Society of Soil Science. This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited. Lanthanides in a Mediterranean soil chronosequence 395 with the source area. This feature can be used in paleoenvironmental studies of sedimentary origin and tectonic setting (Chen et al., 2014; Och et al., 2014), and in soil studies such as pedogenic tracers (Laveuf & Cornu, 2009). The concentrations of REE in soil have been shown to depend not only on the lithology over which they develop, but also there are soil processes that induce internal fractionation or anomalies (Laveuf & Cornu, 2009). The Ln patterns, where the abundance of each Ln relative to that of a chondrite or shale is plotted on a logarithmic scale against the atomic number, or the geochemical ratios between lanthanides (e.g. HREE/LREE, lanthanum/ytterbium (La/Yb), samarium/ytterbium (Sm/Yb), and so on, or the cerium and europium anomalies (Ce/Ce* and Eu/Eu*, respectively)) are used for studying the provenance of geological materials (as fingerprints) and for analysing pedogenic intensity (Rollinson, 1993; Moreno et al., 2006; Mongelli et al., 2014). In soil lanthanide dynamics, another process to add to inheritance and pedogenic action is aeolian contribution. Aeolian processes are common in Mediterranean soils (Delgado et al., 2003). Relatively little is known about lanthanide behaviour in soil (Chen et al., 2014); therefore, their dynamics in different soil environments need to be analysed (Laveuf et al., 2012). Studies of REE concentrations of granulometric fractions of soils, including sands (2000–50 μm) (Aide & Smith-Aide, 2003; Marques et al., 2011) or in free forms, or in soil chronosequences are even more scarce (Chang et al., 2016; Martín-García et al., 2016). The aim of the present study was to examine lanthanide concentrations in the fine sand and clay fractions and the free forms of clay from a soil chronosequence from the River Guadalquivir (southern Spain) (a soil chronosequence is a series of soils that differ in their degree of profile development because of differences in age, while other soil-forming factors remain relatively constant). Other novel aspects investigated in this study are: (i) the effect of soil age, including the formulation of chronofunctions, (ii) the correlations between concentrations, (iii) the use of lanthanides as fingerprints of provenance compared with geological samples from the same soil zone and (iv) the contribution to the soil of lanthanides from aeolian materials. In previous studies we have shown how the soils of the Guadalquivir behave like an ideal chronosequence, in which a considerable number of components and properties fitted significantly to chronofunction equations (Calero et al., 2008, 2009, 2013). In addition, Martín-García et al. (2016) have studied the geochemistry of the clay fraction (including some aspects of lanthanides). The present study can be included in the collection of soil chronofunction studies, which, at present, are few. Materials and methods Setting and soils Geographically, the Guadalquivir River (640-km long) drains an area of 68 300 km2. It rises in the Baetic Cordillera at a height of 1400 m before flowing into the Atlantic Ocean. It is the most important fluvial system in the southern Iberian Peninsula. From a geological point of view, the Guadalquivir-Cenozoic Basin was developed between the Iberian Massif (passive margin) to the north and the Baetic Cordillera (active margin) to the south (Figure 1). From a sedimentological point of view their fluvial alluvia are gravels with some stone-free sandy silt layers. The source rocks for this alluvium are lithologically diverse and include: to the north, igneous rocks (such as granite, granodiorite, rhyolite, tonalite, andesite, gabbro and intrusive rocks) and metamorphic rocks (mainly shales) from the Iberian Massif (Central Iberian Zone, mainly Los Pedroches batholith and Santa Elena pluton, and the Iberian Massif) (Larrea et al., 1992, 1994, 1995; Carracedo et al., 1997; Pin et al., 2002; Pascual et al., 2008); to the south and west, sedimentary materials such as limestones, marly limestones, marls and dolomites from the External Baetics Zone of the Baetic Cordillera (Martínez-Ruiz, 1994), metasedimentary rocks (schist and gneiss) from the Internal Baetics Zone (Torres-Ruiz et al., 2003) and Quaternary sediments from the Guadalquivir Depression, a Cenozoic Basin (Jiménez-Espinosa et al., 2016). The study area is in the middle reaches of the Guadalquivir River, near the town of Andújar on a transect of 3.7 km along the river between 3∘50′–4∘3′W and 38∘0′–38∘2′N(Figure1).Thesoils selected (Table 1) developed on four Quaternary terrace surfaces (P1, P2, P3 and P4: Luvisols and Calcisols) and a floodplain (P5: Fluvisol) with ages ranging from 600 to 0.3 ka (Calero et al., 2008). Fresh point bar sediments (PM) in the river were also selected. Currently, the climate is hot in the Mediterranean with mean annual rainfall of 650 mm and a mean annual temperature of 18 ∘C. The vegetation is mainly anthropogenic because the flat surfaces have been cultivated since time immemorial (nowadays olive groves, wheat and cotton). The solum of the older terrace soils (pre-Holocene soils: P1, P2 and P3) (Table 1) shows Bt horizons (with clay illuviation features such as clay cutans), red Munsell colours, relatively deep thickness, clayey textures (>30% clay) and evidence of leaching of carbonates (and accumulation, in P2). Thus, the older soils have the largest values of Harden’s profile development index (between 44.8 for P1 and 39.6 for P3; Table 1). In the Holocene soil P4, brunification and some leaching and accumulation of carbonates has also been detected. The soil P5 had no evident evolution features. In the fine sand fraction (Table 1), quartz was the main mineralogical component in P1, P2 and P3 (≥58%, mean value), whereas the carbonates, calcite and dolomite were the main components in P4 andP5(≥44%, mean value of total carbonates). Other phases present were phyllosilicates (illite, brammallite, chlorite, kaolinite and various mixed-layer phases), feldspars (potassium feldspar and plagioclases, both abundant in P3) and iron (hydr)oxides (goethite and haematite). The point bar sediment PM showed a more balanced composition of quartz and carbonates (30 and 40%, respectively). Materials and lanthanide analyses Lanthanide concentrations of the fine sand fraction (50–250 μm) of 24 samples (belonging to soil horizons and point bar sediment, PM) were determined by inductively coupled plasma–mass © 2018 The Authors. European Journal of Soil Science published by John Wiley & Sons Ltd on behalf of British Society of Soil Science European Journal of Soil Science,70, 394–410 396 J. M. Martín-García et al. Figure 1 Location of the study area, geology of the Guadalquivir catchment, position of fluvial terrace levels (Terraces 1 to 4 and flood plain), topographic profile (A–A′) and soil sampling (P1 to P5 and PM). spectrometry (ICP–MS) using an Agilent 7700x (Santa Clara, CA, USA) instrument at the Natural History Museum (London, UK) after lithium metaborate fusion in a Pt–Au crucible, and the resulting flux was dissolved in 10% HNO3. Calibration was performed using certified reference materials (CRM) prepared in the same way. Further analytical details are given in Gregory et al. (2017). As study material, we also used the concentrations of lanthanides from the clay fraction and the free forms of clay after extraction with citrate-dithionite-bicarbonate (FFclay-CDB) or with ammonium oxalate (FFclay-Ox), measured previously by Martín-García et al. (2016). The FFclay-CDB is conventionally assumed to be a measure of the total pedogenic free forms (crystalline and poorly crystalline forms), whereas FFclay-Ox is a measure of poorly crystalline forms; mainly iron, but with appreciable quantities of Al and Ti. We grouped lanthanides following Rollinson (1993) into light (LREE: La to Nd), medium (MREE: Sm to Dy) and heavy rare earth elements (HREE: Ho to Lu). Lanthanide concentrations were normalized to (i.e. divided by) the CI chondrite, considered to represent the bulk earth composition, of McDonough & Sun (1995), and then the Ce/Ce* and Eu/Eu* anomalies were calculated (Ce/Ce* =CeN/(LaN×PrN)1/2;Eu/Eu*=EuN/(SmN×GdN)1/2;the subscript Nshows that the value was normalized by the chondrite used. It makes sense to calculate both anomalies because Eu3+and Ce3+might be in another valency (Eu2+and Ce4+) and thus be involved in different reactions from those of the rest of the trivalent lanthanides (Ln3+) (i.e. separate from the group behaviour). The ratios LaN/YbN,Sm N/YbN,HREE N/LREENand MREEN/LREEN were also calculated; all establish the degree of fractionation of LREE from MREE and HREE (La is a representative of LREE, Sm of MREE and Yb of HREE) during geochemical processes. Statistical analysis The statistical analysis was carried out using the IBM SPSS v.22.0 software package. The Kolmogorov–Smirnov and Shapiro–Wilk tests were used to determine the normality of data, and the results were considered statistically significant if Pwas less than 0.05. Statistical analyses were carried out after data were transformed © 2018 The Authors. European Journal of Soil Science published by John Wiley & Sons Ltd on behalf of British Society of Soil Science European Journal of Soil Science,70, 394–410 Lanthanides in a Mediterranean soil chronosequence 397 Table 1 Soil characteristicsa. Mean values and standard deviation (in parentheses) Fine earth fraction (<2 mm) Clay fraction (<2μm) Mineralogy (XRD) / % Fine sand (50–250 μm) Clay (<2μm) Profile (Terrace No) Soil classificationb Age / ka PDI Clay /% Fine sand / % OC /% pH CEC / cmol+kg−1 FFCDB /% CaCO3 eq/% FFclay-CDB /% FFclay-Ox / % phy qz fd feox ca do phy qz fd feox ca P1 (Terrace 1) Cutanic Luvisol/ Palexeralf 600c44.8 29.4 (7.2) 35.8 (9.6) 0.24 (0.14) 7.4 (0.4) 8.5 (1.8) 3.64 (0.44) 0.5 (0.9) 5.61 (0.83) 0.38 (0.06) 29 (9) 61 (10) 6 (3) 3 (1) 1 (0) 86 (1) 8 (1) 1 (1) 5 (1) 0 (0) P2 (Terrace 2) Lixic Calcisol/ Haploxeralf 300c44.3 42.9 (5.5) 21.4 (13.4) 0.32 (0.23) 7.9 (0.1) 15.1 (2.8) 3.24 (0.36) 16.9 (19.4) 3.66 (0.35) 0.38 (0.06) 18 (6) 58 (8) 12 (10) 3 (1) 9 (10) 87 (1) 5 (1) 2 (1) 4 (1) 2 (1) P3 (Terrace 3) Cutanic Luvisol/ Haploxeralf 70c39.6 30.7 (7.5) 36.4 (9.2) 0.22 (0.09) 7.6 (0.2) 10.7 (7.7) 1.90 (0.34) 0 (0.0) 3.06 (0.28) 0.53 (0.10) 6 (2) 58 (10) 31 (10) 2 (1) 3 (3) 89 (2) 6 (1) 2 (1) 3 (1) 0 (1) P4 (Terrace 4) Haplic Calcisol/ Calcixerept 7d26.8 24.5 (8.5) 29.9 (6.8) 0.37 (0.25) 8.2 (0.4) 8.9 (6.1) 1.80 (0.48) 24.2 (7.1) 1.60 (0.43) 0.33 (0.05) 14 (5) 30 (15) 9 (7) 3 (1) 14 (5) 30 (18) 87 (3) 3 (1) 2 (1) 2 (1) 6 (3) P5 Flood plain Haplic Fluvisol/ Xerofluvent 0.3d21.2 17.5 (3.4) 43.0 (8.2) 0.41 (0.19) 8.0 (0.1) 6.9 (3.3) 1.27 (0.11) 39.0 (1.9) 1.15 (0.15) 0.45 (0.11) 20 (8) 26 (5) 6 (3) 2 (1) 29 (3) 17 (5) 88 (1) 3 (2) 1 (0) 1 (1) 7 (0) PMe016.321.61.768.48.7 0.7823.51.06 0.601730103 337853219 aFrom Calero et al. (2009) and Martín-García et al. (2016). bWorld Reference Base for Soil Resources/Soil Taxonomy. cPre-Holocene. dHolocene. eParent material: Fluvial sediment ‘point bar’. PDI, profile development index (Harden, 1982); OC, organic carbon; CEC, cation exchange capacity; FFCDB, citrate-ditionite-bicarbonate extractable free forms in the fine earth fraction (Fe2O3+Al2O3); FFclay-CDB, citrate-dithionite-bicarbonate extractable free forms in the clay fraction (Fe2O3+Al2O3+TiO2); FFclay-Ox, ammonium oxalate extractable free forms in the clay fraction (Fe2O3+Al2O3+TiO2); CaCO3eq, calcium carbonate equivalent; phy, phyllosilicates (illite, smectite, mixed-layer phases, kaolinite, chlorite, brammallite); qz, quartz; fd, feldspar; feox, iron (hydr)oxides (goethite, hematite); ca, calcite; do, dolomite. © 2018 The Authors. European Journal of Soil Science published by John Wiley & Sons Ltd on behalf of British Society of Soil Science European Journal of Soil Science,70, 394–410 398 J. M. Martín-García et al. logarithmically if this was necessary. The matrix of Pearson’s product-moment correlation coefficients (r) and, in some cases, the coefficients of determination (R2) were obtained. The coefficient of determination (R2) was obtained to determine the fit of the least squares regressions. Results and discussion Lanthanides in the fine sand fraction The series of lanthanides in fine sand showed very variable values, ranging from around 0.05 to 65 mg kg−1(Table 2). The order of abundance was Ce >La >Nd >Pr >Sm >Gd > Dy >Er >Yb >Eu >Ho >Tb >Tm >Lu, identical to that of the mean of the Earth’s crust (Rollinson, 1993; British Geological Survey, 2011), so that ΣLREE >ΣMREE >ΣHREE (Table 3). Lanthanide concentration increased with depth in the profile (Table 2). The P1 horizons contained most ΣLn (Table 3). The ΣLREE increased with age (P1 >P2 >P3 >P4 >P5 >PM) and profile P4 was the richest in ΣHREE and Dy (e.g. horizon 4C2 had the largest concentration). The ratios MREEN/LREENand HREEN/LREENwere smaller in pre-Holocene than Holocene soils and PM (Table 3). The concentrations of lanthanides (ΣLn) were related to those of phyllosilicates, as proposed by Mongelli et al. (2014). In the fine sand of the present study, this is determined as: ΣLn (mgkg−1)=1.85 ×Phyllosilicates (%) +59.08 (n=24;r=0.538;P<0.01). In addition, the abnormally large concentrations of Dy and HREE in 4C2 of profile P4 (Table 2) reaffirm the presence of the lithological discontinuity detected morphologically by Calero et al. (2008, 2009). A possible explanation might be a change in the mineralogical composition of the major species (phyllosilicates, quartz, feldspars, iron oxides, calcite and dolomite) compared with the other horizons of the profile. However, this mineralogical change was not detected (table 4 on page 471 of Calero et al., 2009). Therefore, it must be assumed that the change is in the minority mineral phases (<1%), which are those having an important role in lanthanide concentration (Kerr & Rafuse, 2012). Thus, the excess of Dy and HREE in geologic materials might be a result of the presence of minerals such as thortveitite, with the formula (Sc,Y)Si2O7, which can show detectable concentrations of Dy, Ho, Er, Tm, YbandLu(Guastoniet al., 2012), or xenotime, with the formula (HREE,Y)PO4. However, verification of this would be beyond the scope of the present study. The chondrite-normalized profiles (Figure 2a) always had values >1 (i.e. larger quantities than in the reference meteorite), a common tendency in soil materials (Hu et al., 2006). Figure 2(a) also indicates a pronounced relative abundance of LREE, shown by a steep slope that flattens out in such a way that after Ho (region of the HREE) it is almost horizontal. The P1 horizons, with more lanthanides, occupy the highest positions on the graph and the 4C2 horizon of P4 is V-shaped because of its large Dy and HREE concentrations. No notable Ce/Ce* anomaly was detectable (Figure 2a, Table 3), with values very close to unity (all horizons between 0.93 and 1.04). The mean values of Ce/Ce* per profile increased with soil age, probably as a result of alteration processes in the soil material (Huang & Gong, 2001). In the present study, the alteration could be related to decarbonation of the fine sand (decrease in calcite and dolomite content through leaching) (Table 1), as shown by the moderate negative correlation in fine sand between Ce/Ce* and the sum of calcite +dolomite, % (r=−0.530, n=22, P<0.01), which accords with the results of Wen et al. (2014), in their case with r=−0.403 and P<0.01. According to its lanthanides, the fine sand fraction has evolved geochemically rather than being inert. Lanthanides in the clay fraction and clay free forms In the clay fraction, the order of abundance of ΣLn (mean values per profile in mg kg−1)is:P3>P2 >P4 >P1 >P5 >PM (Table 3), with no obvious trend regarding age or when compared with the respective fine sand (Table 3). Aide & Smith-Aide (2003) and Marques et al. (2011) reported that the lanthanides are concentrated in the fine fractions, <50 μm (silt and clay); in the present study, this was not clear in P1 and P5. This might be a result of the differences in phyllosilicate content because these were always larger in the clay (Table 1), and in P1 and P5 this would suggest searching for the presence of lanthanide-rich minerals (e.g. zircon) in fine sand; again, this is beyond the scope of the present study. Moreover, in the clay fraction, as occurred in the fine sand, there was a relation between ΣLn and phyllosilicates (n=35, r=0.566, P<0.001). Considering the complete population (clay +fine sand), this correlation was not significant. The clay of the pre-Holocene soils (P1, P2 and P3) contained more ΣHREE than that of two of the Holocene soils (P4 and P5) and PM. Furthermore, the clay of these pre-Holocene soils also had more ΣHREE than the corresponding fine sand (Table 3). The clay free forms (FFclay-CDB and FFclay-Ox) generally contained fewer lanthanides (ΣLn) than the fine sand and clay fractions (Table 3). However, calculation of ΣLn in FFclay-CDB, assuming that all proceed from the mineral phases that constitute the free forms (in FFclay-CDB it was mainly goethite, haematite and poorly crystalline forms of Fe; in Feclay-Ox it was poorly crystalline iron, mostly ferrihydrite; Martín-García et al., 2016), provided striking new evidence of REE accumulation in iron free forms. Thus, FFclay-CDB of P1, with ΣLn of 39.82 mg kg−1attributable to 5.61% of FFclay-CDB (Fe2O3+Al2O3+TiO2; Table 1), suggests that iron (hydr)oxides (FFclay-CDB)hadΣLn of 709.8 mg kg−1. When the calculation was carried out with PM (1.06% FFclay-CDB (Fe2O3+Al2O3+TiO2; Table 1) and 9.60 mg kg−1of ΣLn) the goethite +haematite had 905.7 mg kg−1. These values of ΣLn can be attributed to the special characteristics of these iron (hydr)oxides: small particle size and neoformed in the soil, absorbing Ln at their surface, which might even have been buried during the growth of the iron (hydr)oxide. © 2018 The Authors. European Journal of Soil Science published by John Wiley & Sons Ltd on behalf of British Society of Soil Science European Journal of Soil Science,70, 394–410 Lanthanides in a Mediterranean soil chronosequence 399 Table 2 Lanthanide content (mg kg−1) in the soil fine sand fraction (50–250 μm) La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu P1 Ap 24.93 53.69 6.37 23.03 4.14 0.66 2.92 0.35 1.87 0.32 0.97 0.14 0.93 0.15 Bt 29.55 64.30 7.58 28.09 5.29 0.93 4.03 0.52 2.97 0.51 1.49 0.21 1.50 0.22 Btg2 31.78 66.50 7.97 29.82 5.66 1.05 4.22 0.53 3.05 0.53 1.56 0.23 1.56 0.22 2BCtg 26.21 54.91 6.69 25.06 4.91 0.95 3.51 0.42 2.37 0.40 1.29 0.17 1.29 0.19 4C 29.86 61.30 7.32 26.78 5.11 0.93 3.54 0.44 2.46 0.41 1.23 0.18 1.27 0.19 Meana28.85 (2.81) 60.92 (5.66) 7.29 (0.65) 27.02 (2.63) 5.14 (0.57) 0.94 (0.15) 3.75 (0.51) 0.47 (0.07) 2.64 (0.48) 0.45 (0.09) 1.36 (0.23) 0.19 (0.04) 1.37 (0.25) 0.20 (0.03) P2 Ap 10.79 21.59 2.51 9.08 1.66 0.34 1.13 0.14 0.8 0.15 0.42 0.06 0.45 0.07 Btg1 18.82 37.44 4.66 17.06 3.18 0.64 2.53 0.34 1.95 0.38 1.05 0.15 0.99 0.15 Btg2 17.09 32.75 3.73 13.26 2.35 0.50 1.67 0.21 1.23 0.23 0.67 0.11 0.68 0.10 Cmk/Bt 23.76 48.96 6.04 22.56 4.28 0.80 3.20 0.40 2.25 0.46 1.19 0.17 1.09 0.18 Meana17.66 (5.36) 35.20 (11.34) 4.20 (1.49) 15.35 (5.73) 2.83 (1.13) 0.56 (0.20) 2.08 (0.92) 0.26 (0.12) 1.51 (0.66) 0.29 (0.14) 0.80 (0.35) 0.12 (0.05) 0.78 (0.29) 0.12 (0.05) P3 Ap1 11.97 22.72 2.96 10.72 2.06 0.37 1.47 0.19 1.12 0.21 0.63 0.09 0.59 0.09 Bt1 17.30 32.51 4.16 15.06 2.95 0.45 1.99 0.27 1.56 0.30 0.94 0.13 0.83 0.13 Bt2 15.00 28.97 3.65 12.75 2.46 0.42 1.77 0.23 1.33 0.25 0.77 0.11 0.71 0.11 Bt3 18.86 38.83 4.75 17.31 3.37 0.47 2.43 0.31 1.83 0.36 1.09 0.15 0.92 0.14 Bt4 17.50 35.79 4.26 15.35 2.87 0.50 2.01 0.27 1.46 0.28 0.82 0.12 0.74 0.11 2Bt6 23.32 47.41 5.69 20.24 3.86 0.51 2.94 0.44 2.70 0.53 1.54 0.23 1.41 0.22 Meana18.01 (3.81) 35.78 (8.49) 4.41 (0.93) 15.85 (3.35) 3.05 (0.64) 0.46 (0.05) 2.19 (0.52) 0.30 (0.09) 1.76 (0.56) 0.34 (0.11) 1.02 (0.32) 0.15 (0.05) 0.91 (0.29) 0.14 (0.05) P4 Ap1 11.21 21.74 2.80 10.43 2.10 0.45 1.63 0.22 1.34 0.26 0.78 0.11 0.65 0.10 Bwk1 12.53 24.01 3.06 11.32 2.23 0.47 1.76 0.24 1.47 0.29 0.84 0.12 0.71 0.11 2Bwk2 26.43 54.91 6.47 23.59 4.38 0.54 3.02 0.37 2.11 0.38 1.17 0.19 1.11 0.17 4C2 13.33 26.26 3.26 12.25 2.52 0.54 2.76 0.54 4.41 1.01 3.50 0.53 3.45 0.54 Meana16.80 (7.09) 33.77 (15.56) 4.12 (1.73) 15.21 (6.17) 2.96 (1.06) 0.51 (0.05) 2.48 (0.70) 0.38 (0.15) 2.71 (1.43) 0.57 (0.35) 1.89 (1.30) 0.29 (0.20) 1.81 (1.33) 0.28 (0.21) P5 Ap 16.03 31.08 4.02 15.25 3.03 0.61 2.57 0.34 2.09 0.38 1.13 0.15 0.94 0.17 2C1 12.51 24.40 3.13 11.98 2.38 0.51 1.96 0.26 1.70 0.32 0.97 0.13 0.81 0.12 4C3 16.78 34.21 4.07 15.26 2.99 0.58 2.54 0.34 2.03 0.39 1.17 0.16 1.03 0.16 6C5 15.39 29.65 3.74 13.95 2.72 0.53 2.40 0.33 2.04 0.39 1.16 0.16 1.01 0.15 Meana15.09 (1.87) 29.72 (4.09) 3.73 (0.43) 14.09 (1.55) 2.78 (0.30) 0.56 (0.05) 2.36 (0.28) 0.31 (0.04) 1.95 (0.18) 0.37 (0.03) 1.10 (0.09) 0.15 (0.01) 0.94 (0.10) 0.15 (0.02) PM 10.41 19.98 2.58 9.76 2.01 0.43 1.56 0.20 1.23 0.24 0.72 0.10 0.61 0.09 aWeighted to horizon thickness (standard deviation in parentheses). © 2018 The Authors. European Journal of Soil Science published by John Wiley & Sons Ltd on behalf of British Society of Soil Science European Journal of Soil Science,70, 394–410 400 J. M. Martín-García et al. Table 3 Total content and selected geochemical ratios of lanthanides. Mean profile values and standard deviation (in parentheses). Soil fine sand and soil clay fractions and free forms of the soil clay fraction extracted with citrate-dithionite-bicarbonate (FFclay-CDB) and oxalate (FFclay-Ox). ΣLn/mg kg−1ΣLREE/mg kg−1ΣMREE/mg kg−1ΣHREE/mg kg−1MREEN/LREENHREEN/LREENEu/Eu* Ce/Ce* Soil fine sand (50–250 μm) (n=24) P1 140.59 (12.87)124.08 (11.65)12.94 (1.75)3.57 (0.63) 0.26 (0.02) 0.12 (0.01) 0.65 (0.05) 1.02 (0.02) P2 81.77 (15.34)72.40 (23.85)7.24 (3.01)2.12 (0.88) 0.25 (0.02) 0.12 (0.02) 0.73 (0.06) 0.99 (0.02) P3 84.37 (14.15)74.04 (16.54)7.76 (1.82)2.56 (0.82) 0.26 (0.01) 0.14 (0.02) 0.55 (0.07) 0.97 (0.04) P4 83.80 (35.71)69.91 (30.55)9.05 (2.69)4.85 (3.39) 0.31 (0.10) 0.28 (0.26) 0.61(0.13) 0.97 (0.04) P5 73.29 (9.86)62.63 (7.86)7.96 (0.83)2.70 (0.26) 0.31 (0.01) 0.17 (0.01) 0.67 (0.04) 0.96 (0.03) PM 49.92 42.73 5.43 1.76 0.32 0.16 0.74 0.93 Soil clay (<2μm) (n=35) P1 86.99 (12.31)71.75 (10.71)11.10 (1.25)4.14 (0.37) 0.39 (0.01) 0.24 (0.02) 0.67 (0.02) 0.94 (0.01) P2 105.23 (15.80)90.73 (13.20)10.50 (2.08)4.00 (0.53) 0.30 (0.01) 0.19 (0.01) 0.66 (0.01) 1.15 (0.08) P3 143.01 (26.60)123.25 (23.61)14.86 (2.42)4.90 (0.69) 0.30 (0.01) 0.16 (0.01) 0.65 (0.01) 1.00 (0.05) P4 97.84 (34.20)83.67 (29.50)10.68 (3.67)3.49 (1.05) 0.32 (0.01) 0.17 (0.01) 0.63 (0.04) 1.04 (0.03) P5 68.81 (17.10)58.63 (14.79)7.58 (1.75)2.60 (0.56) 0.33 (0.01) 0.18 (0.01) 0.69 (0.01) 0.98 (0.01) PM 51.46 43.72 5.74 2.00 0.33 0.19 0.71 0.99 FFclay-CDB (n=35) P1 39.82 (10.08)31.07 (8.49)6.92 (1.33)1.83 (0.27) 0.58 (0.07) 0.25 (0.04) 0.75 (0.02) 1.09 (0.11) P2 45.74 (13.45)38.10 (10.92)5.87 (2.00)1.77 (0.54) 0.44 (0.01) 0.21 (0.01) 0.71 (0.03) 1.74 (0.33) P3 67.41 (12.30)54.91 (10.20)9.45 (1.64)3.05 (0.41) 0.47 (0.04) 0.24 (0.02) 0.71 (0.01) 1.39 (0.07) P4 13.64 (3.09)10.83 (2.62)2.05 (0.36)0.76 (0.13) 0.53 (0.04) 0.31 (0.02) 0.74 (0.03) 1.50 (0.19) P5 14.66 (6.03)11.58 (4.97)2.29 (0.84)0.79 (0.23) 0.51 (0.04) 0.29 (0.05) 0.75 (0.04) 1.05 (0.03) PM 9.60 7.66 1.44 0.50 0.49 0.27 0.75 1.03 FFclay-Ox (n=35) P1 1.76 (0.86)0.92 (0.54)0.55 (0.22)0.29 (0.11) 1.39 (0.46) 1.34 (0.43) 0.65 (0.35) 0.86 (0.09) P2 1.22 (0.84)0.61 (0.53)0.36 (0.20)0.25 (0.11) 1.58 (1.30) 1.91 (2.21) 0.88 (0.36) 1.83 (1.09) P3 3.93 (1.12)2.26 (0.74)0.98 (0.26)0.69 (0.18) 1.09 (0.20) 1.35 (0.18) 0.55 (0.06) 1.40 (0.36) P4 1.02 (0.37)0.66 (0.25)0.24 (0.10)0.13 (0.06) 0.83 (0.31) 0.84 (0.58) 0.24 (0.09) 1.24 (0.46) P5 0.61 (0.23)0.28 (0.14)0.17 (0.06)0.16 (0.05) 1.41 (0.32) 2.35 (1.65) 1.08 (0.43) 0.92 (0.56) PM 0.58 0.25 0.20 0.13 2.11 2.89 0.34 2.22 Ln, La to Lu; LREE, La to Nd; MREE, Sm to Dy; HREE, Ho to Lu; the suffix “N” shows that the value normalized to chondrite was used; Eu/Eu* =EuN/(SmN×GdN)1/2; Ce/Ce* =CeN/(LaN×PrN)1/2;FF clay-CDB, citrate-dithionite-bicarbonate extractable free forms in clay fraction; FFclay-Ox, ammonium oxalate extractable free forms in clay fraction. This proves that iron (hydr)oxides act as lanthanide scavengers. Our values were small considering that Onac et al. (1997) reported Σ(La, Ce, Sm, Nd) greater than 2000 mg kg−1in coatings of ferromanganese (hydr)oxides. The ΣLn in the clay free forms (FFclay-CDB and FFclay-Ox) tended to increase with soil age because they were more abundant in pre-Holocene soils than in Holocene soils and PM (Table 3). The chondrite-normalized patterns (Figure 2b) showed that in most cases the clay was within the range of concentrations of fine sand. The profiles of FFclay-CDB (Figure 2c) differentiate clearly between the pre-Holocene soils (P1, P2, P3) and the Holocene soils (P4, P5) and PM, which had smaller concentrations. The ratio HREEN/LREENin the clay fraction (Table 3) was greater than in the fine sand (except in P4). Laveuf & Cornu (2009) stated that during pedogenesis the LREE are less readily complexed by fluids than the HREE and that the latter accumulate in alteration products such as phyllosilicates, which are more abundant in the clays than in the fine sand. Furthermore, the HREEN/LREEN index was greater in FFclay-CDB and FFclay-Ox (neoformed iron (hydr)oxides) than in fine sand or clay, which was in accord with Pédrot et al. (2015), who reported that the iron (hydr)oxides precipitated during alteration have a greater affinity for HREE than for LREE. This all suggests fractionation of the Ln by granulometric fractions (and mineralogy), with the HREE being concentrated in the clay, FFclay-CDB and FFclay-Ox. The values of the Ce/Ce* anomaly of the clay fraction (0.94–1.14) (Table 3, Figure 2a) were also similar to those of the fine sand. In FFclay-CDB all these values were positive (1.03–1.62), indicating a relative accumulation of Ce in the iron (hydr)oxides also reported by Pédrot et al. (2015); in FFclay-Ox the range was wider (0.86–2.22) and erratic again. Behaviour of lanthanides in relation to soil horizon evolution and time The lanthanide concentrations exhibited very different behaviours in the fine sand and clay fractions with regard to the morphological evolution of the soil horizons, measured with the horizon development index (HDI) (Harden, 1982) (Table 4). Positive linear correlations with P<0.01 were typical of the clay fraction, whereas © 2018 The Authors. European Journal of Soil Science published by John Wiley & Sons Ltd on behalf of British Society of Soil Science European Journal of Soil Science,70, 394–410 Lanthanides in a Mediterranean soil chronosequence 401 (a) (b) (c) (d) (e) (f) (g) (h) (i) Figure 2 Chondrite-normalized concentrations of lanthanides (logarithmic scale) in: (a) the soil fine sand fraction (50–250 μm) (all horizons; this study), (b) the soil clay fraction (<2μm) (mean profile, this study), (c) free forms from the soil clay fraction (mean profile, this study) extracted with citrate-dithionite-bicarbonate (FFclay-CDB), (d) free forms from the soil clay fraction (mean profile, this study) extracted with oxalate (FFclay-Ox), (e) acid igneous and magmatic-like rocks from the Guadalquivir catchment (Larrea et al., 1992, 1994, 1995; Carracedo et al., 1997; Pascual et al., 2008), (f) alkaline igneous rocks from the Guadalquivir catchment (Larrea et al., 1995; Pin et al., 2002), (g) sedimentary rocks from the Guadalquivir catchment (Martínez-Ruiz, 1994; Jiménez-Espinosa et al., 2016), (h) metasedimentary rocks from the Guadalquivir catchment (Torres-Ruiz et al., 2003) and (i) Sahara–Sahel materials (Moreno et al., 2006) and Spanish topsoil (Locutura et al., 2012). The shaded area (b to i) and the area with horizontal lines (c to i) enclose the upper and lower margins of the mean values per profile of the fine sand and clays (a and b). © 2018 The Authors. European Journal of Soil Science published by John Wiley & Sons Ltd on behalf of British Society of Soil Science European Journal of Soil Science,70, 394–410 402 J. M. Martín-García et al. Table 4 Matrix of linear correlations (Pearson’s r) between lanthanides content and horizon development index (HDI)a Lanthanide content Soil fine sandb(n=24) Soil claya (n=35) FFclay-CDBa (n=35) FFclay-Oxa (n=35) La 0.314 0.557 0.718 0.367 Ce 0.317 0.579 0.765 0.483 Pr 0.297 0.565 0.745 0.442 Nd 0.257 0.551 0.746 0.409 Sm 0.192 0.572 0.766 0.503 Eu 0.031 0.526 0.745 0.467 Gd 0.020 0.567 0.741 0.516 Tb −0.077 0.564 0.746 0.454 Dy −0.203 0.589 0.745 0.527 Ho −0.227 0.594 0.740 0.373 Er −0.267 0.614 0.730 0.403 Tm −0.227 0.656 0.724 0.431 Yb −0.213 0.705 0.746 0.484 Lu −0.219 0.727 0.741 0.517 ΣLn 0.252 0.575 0.762 0.537 ΣLREE 0.304 0.569 0.761 0.461 ΣMREE 0.007 0.571 0.753 0.519 ΣHREE −0.240 0.656 0.739 0.454 Statistical significance: P<0.05;P<0.01;P<0.001. aHDI and lanthanides values from Martín-García et al. (2016). bLanthanides values from Table 2. FFclay-CDB, citrate-dithionite-bicarbonate extractable free forms in clay fraction; FFclay-Ox, ammonium oxalate extractable free forms in clay fraction. there was no significant correlation with the fine sand. Thus, lanthanide concentration in clay increases with horizon evolution, particularly in HREE, which are those with the strongest correlations (P<0.001; r=0.656, 0.705, 0.727 and 0.656 for Tm, Yb, Lu and ΣHREE, respectively). The clay free forms FFclay-CDB showed the same behaviour as the clay fraction, with the correlations being even more significant (with P<0.001), possibly because of the previously mentioned role of iron (hydr)oxides (goethite and haematite, principal constituents of the FFclay-CDB) as scavengers of the lanthanides liberated during alteration of the soil minerals. The ratio Ce/ΣLn (Pédrot et al., 2015) in FFclay-CDB was correlated (P<0.05) with HDI (r=0.416; n=35), suggesting the relative enrichment of Ce in FFclay-CDB in the most morphologically evolved horizons. Evidence has already been provided to illustrate the dependence of the lanthanides in the present study (properties ΣLn, ΣLREE, ΣHREE, Ce/Ce*, MREEN/LREENand HREEN/LREEN)onage groups of soils: pre-Holocene (P1, P2 and P3) and Holocene (P4, P5 and PM). To quantify these relations better, we calculated the correlation matrix of lanthanide concentration with soil age (Table 5). The behaviour was different: the fine sand showed a linear relation with time (y=ax +b) and a quadratic relation with time (y=ax2+bx +c) in LREE and part of MREE (La to Gd), and ΣLn, whereas the clay fraction was fitted well by logarithmic functions (y=alnx+b) in HREE and another part of MREE (Tb to Lu) and ΣHREE. For the fine sand, this suggests that the concentrations of lanthanides from La to Gd did not attain a stable state (identified by the logarithmic model). On the other hand, the concentrations in clay of lanthanides from Tb to Lu did attain a stable state. Furthermore, the chronofunctions with strong correlations spanned from La to Gd in fine sand and were present from Gd (not included) in the clay, suggesting the ‘gadolinium breaking effect’ (Chi et al., 2006). The latter is the infringement of the monotonic change of properties of lanthanide compounds according to the atomic number, attributed to a variation in the electron configuration of the lanthanides occurring in the gadolinium. When the problem of soil age is considered from the point of view of LREE and HREE fractionation, estimated by the indices HREEN/LREENand LaN/YbN(Table 5), it can appear that this process depends markedly on age, with the HREE concentration increasing with time, for example in soil clay: LaN∕YbN=−5.2 ×10−3×age (ka)+10.39; n=6;R2=0.870;P<0.01. However, a stable state was not attained for either fine sand or clay because the correlations never fitted the logarithmic functions. Relations between lanthanide quantities Although concentrations of elements of the lanthanide series were different in all the fractions analysed (Tables 2 and 3, Figure 2a–d), many changed in a parallel way in the samples showing a strong positive collinearity (large rCLC;whererCLC are the correlation coefficients between ‘couples of lanthanide concentrations’) (Figure 3). In the fine sand, the rCLC varied between 0.998 (P<0.001) and 0.477 (P<0.001). In the clay fractions (total clay, FFclay-CDB and FFclay-Ox)rCLC was even larger, ranging from 0.998 to 0.896 (both with P<0.001). Salminen (2005) stated that in European soils ‘all REE in soil are strongly correlated among themselves, with all correlation coefficients higher than 0.7’. However, in the present study, more than a third of the rCLC of the fine sand were below 0.7. Furthermore, rCLC had (Figure 3) the largest values between couples of adjacent lanthanides in the periodic table (e.g. 60Nd vs. 61Pr: 0.998 in fine sand, >0.945 in clay, FFclay-CDB or FFclay-Ox). This fact was previously reported for igneous materials (Kerr & Rafuse, 2012) and shales (Noack et al., 2015), but never demonstrated for soil. In addition, rCLC became progressively weaker with increasing separation between the elements in the periodic table (e.g. in fine sand, rCLC of 57La with the adjacent 58Ce was 0.997, with 66Dy, 0.622 and with 71Lu, at the other extreme, 0.524). This behaviour has never been described previously in geologic materials, and is related to chemical periodicity (adjacent Ln have similar ionic radii, atomic mass and a +3 valence, and will behave similarly in the crystal structure of the minerals). This behaviour of rCLC enables a bivariate diagram to be produced (Figure 4) showing the rCLC of any couple of lanthanides against their differences in standard atomic weight (ΔAr), with a negative collinearity (P<0.001). The Ar(expressed in decimals) was used because it differentiates more © 2018 The Authors. European Journal of Soil Science published by John Wiley & Sons Ltd on behalf of British Society of Soil Science European Journal of Soil Science,70, 394–410 Lanthanides in a Mediterranean soil chronosequence 409 Calero, J., Delgado, R., Delgado, G. & Martín-García, J.M. 2009. SEM image analysis in the study of a soil chronosequence on fluvial terraces of the middle Guadalquivir (southern Spain). European Journal of Soil Science,60, 465–480. Calero, J., Martín–García, J.M., Delgado, G., Aranda, V. & Delgado, R. 2013. A nano-scale study in a soil chronosequence from southern Spain. European Journal of Soil Science,64, 192–209. Carracedo, M., Larrea, F.J., Alonso-Olazabal, A. & Gil-Ibarguchi, J.I. 1997. The relationship between the plutonic intrusions and the dyke swarm in the Los Pedroches batholith (Iberian Massif, Spain): dykes as a paleotectonic and paleostress indicators. Cadernos do Laboratorio Xeolóxico de Laxe,22, 229–246. Chang, C., Li, F., Liu, C., Gao, J., Tong, H. & Chen, M. 2016. Fractionation characteristics of rare earth elements (REEs) linked with secondary Fe, Mn, and Al minerals in soils. Acta Geochimica,35, 329–339. Chen, L.M., Zhang, G.L. & Jin, Z.D. 2014. Rare earth elements of a 1000-year paddy soil chronosequence: implications for sediment provenances. Parent material uniformity and pedological changes. Geoderma, 230, 274–279. Chi, R.A., Dai, Z.X., Xu, Z.G., Wu, Y.X. & Wang, C.W. 2006. Correlation analysis on partition of rare earth in ion-exchangeable phase form weathered crust ores. Transactions of Nonferrous Metals Society of China, 16, 1421–1425. Delgado, R., Martín-García, J.M., Oyonarte, C. & Delgado, G. 2003. Genesis of the terrae rossae of the Sierra Gádor (Andalusia, Spain). European Journal of Soil Science,54, 1–16. Gregory, T., Joy, K.H., Strekopytov, S. & Curran, N.M. 2017. Geochemistry and petrology of howardite Miller Range 11100: a lithologically diverse piece of the Vestan regolith. Meteoritics & Planetary Science,52, 206–224. Guastoni, A., Nestola, F., Ferraris, C. & Parodi, G. 2012. Xenotime-(Y) and Sn-rich thortveitite in miarolitic pegmatites from Baveno, Southern Alps, Italy. Mineralogical Magazine,76, 761–767. Harden, J.W. 1982. A quantitative index of soil development from field descriptions: examples from a chronosequence in central California. Geoderma,28, 1–28. Hu, Z., Haneklaus, S., Sparovek, G. & Schnug, E. 2006. Rare earth elements in soils. Communications in Soil Science and Plant Analysis, 37, 1381–1420. Huang, C. & Gong, Z. 2001. Geochemical implication of rare earth elements in process of soil development. Journal of Rare Earths,19, 57–62. Jiménez-Espinosa, R., Jiménez-Millán, J. & García-Tortosa, F.J. 2016. Upper-Pleistocene terrace deposits in Mediterranean climate: geomorphological and source-rock control on mineral and geochemical signatures (Betic Cordillera, SE Spain). Journal of Iberian Geology,42, 187–200. Kerr, A. & Rafuse, H. 2012. Rare-earth Element (REE) Geochemistry of the Strange Lake Deposits: Implications for Resource Estimation and Metallogenic Models. Report No 12-1. Newfoundland and Labrador Department of Natural Resources, Geological Survey Branch, St. John’s, Canada. Larrea, F.J., Carracedo, M., Cueto, L.A., Quesada, C., Gil-Ibarguchi, J.I., Fernández, F.J. et al. 1992. Petrology and geochemistry of Cardeña-Virgen de La Cabeza pluton (batholith of Los Pedroches). Cadernos do Laboratorio Xeolóxico de Laxe,17, 209–222. Larrea, F.J., Carracedo, M., Ortega, L.A. & Gil-Ibarguchi, J.I. 1994. The Linares granite (Jaén): cartography, petrology and geochemistry. Cadernos do Laboratorio Xeolóxico de Laxe,19, 335–346. Larrea, F.J., Carracedo, M., Ortega, L.A. & Gil-Ibarguchi, J.I. 1995. The Santa Elena stock (Jaén). An intrusion genetically independent from the magmatic association of the Los Pedroches Batolith. Cadernos do Laboratorio Xeolóxico de Laxe,20, 151–166. Laveuf, C. & Cornu, S. 2009. A review on the potentiality of rare earth elements to trace pedogenetic processes. Geoderma,154, 1–12. Laveuf, C., Cornu, S., Guilerme, L.R.G. & Juillot, F. 2012. The impact of redox conditions on the rare earth element signature of redoximorphic features in a soil sequence developed from limestone. Geoderma,170, 25–38. Locutura, J., Bel-lan, A., García-Cortés, A. & Martínez-Romero, S. 2012. Atlas Geoquímico de España.IGME,Madrid. Marques, R., Prudencio, M.I., Dias, M.I. & Rocha, F. 2011. Patterns of rare earth and other trace elements in different size fractions of clays of Campanian–Maastrichtian deposits from the Portuguese western margin (Aveiro and Taveiro Formations). Chemie der Erde,71, 337–347. Martínez-Ruiz, F. 1994. Geoquímica y mineralogía del tránsito Cretácico-Terciario en las Cordilleras Béticas y en la Cuenca Vasco-Cantábrica. Doctoral dissertation, Universidad de Granada, Granada. Martín-García, J.M., Sánchez-Marañón, M., Calero, J., Aranda, V., Delgado, G. & Delgado, R. 2016. Iron oxides and rare earth elements in the clay fractions of a soil chronosequence in southern Spain. European Journal of Soil Science,67, 749–762. McDonough, W.F. & Sun, S. 1995. The composition of the Earth. Chemical Geology,120, 223–253. Meija, J., Coplen, T.B., Berglund, M., Brand, W.A., De Bievre, P., Groning, M. et al. 2016. Atomic weights of the elements 2013 (IUPAC Technical Report). Pure and Applied Chemistry,88, 265–291. Mongelli, G., Paternoster, M., Rizzo, G. & Sinisi, R. 2014. Trace elements and REE fractionation in subsoils developed on sedimentary and volcanic rocks: case study of the Mt. Vulture area, southern Italy. International Journal of Earth Sciences,103, 1125–1140. Moreno, T., Querol, X., Castillo, S., Alastuey, A., Cuevas, E., Herrmann, L. et al. 2006. Geochemical variations in aeolian mineral particles from the Sahara-Sahel Dust Corridor. Chemosphere,65, 261–270. Noack, C.W., Jain, J.C., Stegmeier, J., Hakala, J.A. & Karamalidis, A.K. 2015. Rare earth element geochemistry of outcrop and core samples from the Marcellus Shale. Geochemical Transactions,16, 6. https://doi.org/10 .1186/s12932-015-0022-4. Och, L.M., Müller, B., Wichser, A., Ulrich, A., Vologina, E.G. & Sturm, M. 2014. Rare earth elements in the sediments of Lake Baikal. Chemical Geology,376, 61–75. Onac, B.P., Pedersen, R.B. & Tysseland, M. 1997. Presence of rare-earth elements in black ferromanganese coatings from Vântului Cave (Romania). Journal of Cave and Karst Studies,59, 128–131. Pascual, E., Donaire, T. & Pin, C. 2008. The significance of microgranular enclaves in assessing the magmatic evolution of a high-level composite batholith: a case on the Los Pedroches Batholith, Iberian Massif, Spain. Geochemical Journal,42, 177–198. Pédrot, M., Dia, A., Davranche, M. & Gruau, G. 2015. Upper soil horizons control the rare earth element patterns in shallow groundwater. Geoderma,239–240, 84–96. Pin, C., Liñán, E., Pascual, E., Donaire, T. & Valenzuela, A. 2002. Late Neoproterozoic crustal growth in the European Variscides: Nd isotope and geochemical evidence from the Sierra de Córdoba Andesites (Ossa-Morena Zone, Southern Spain). Tectonophysics,352, 133–151. Rollinson, H.R. 1993. Using Geochemical Data: Evaluation, Presentation, Interpretation. Prentice Hall, Harlow. © 2018 The Authors. European Journal of Soil Science published by John Wiley & Sons Ltd on behalf of British Society of Soil Science European Journal of Soil Science,70, 394–410 410 J. M. Martín-García et al. Salminen, R. (Ed.) 2005. Geochemical Atlas of Europe. Part 1: Background Information, Methodology and Maps. IUGS/IAGC. [WWW document]. URL http://weppi.gtk.fi/publ/foregsatlas/index.php [accessed on 10 July 2018]. Shannon, R.D. 1976. Revised effective ionic radii and systematic studies in interatomic distances in halides and chalcogenides. Acta Crystallographica,A32, 751–767. Torres-Ruiz, J., Pesquera, A., Gil-Crespo, P.P. & Velilla, N. 2003. Origin and petrogenetic implication of turmaline-rich rocks in the Sierra Nevada (Betic Cordillera, southeastern Spain). Chemical Geology,197, 55–86. Wang, G., Li, J., Ravi, S., Van Pelt, R.S., Costa, P.J.M. & Dukes, D. 2017. Tracer techniques in aeolian research: approaches, applications, and challenges. Earth-Science Reviews,170, 1–16. Wen, X.Y., Huang, C.M., Tang, Y., Gong-Bo, S.L., Hu, X.X. & Wang, Z.W. 2014. Rare earth elements: a potential proxy for identifying the lacustrine sediment source and soil erosion intensity in karst areas. Journal of Soils and Sediments,14, 1693–1702. © 2018 The Authors. European Journal of Soil Science published by John Wiley & Sons Ltd on behalf of British Society of Soil Science European Journal of Soil Science,70, 394–410